DRHVLs for internal combustion engines are well known as elements of valve trains disposed between a valve stem or pushrod and an associated engine cam, whereby the engine valve associated with the DRHVL may be selectively activated and deactivated.
The hydraulic lash compensation mechanism of such lifters is similar to the hydraulic compensation mechanism of a conventional lifter. As known in the art, the hydraulic lash compensation mechanism is used to take up lash in the valve train and includes a piston and a plunger defining a low pressure oil chamber and a high pressure oil chamber separated by a one-way check valve that limits the flow of oil from the high pressure to low pressure chambers when the associated valve is open. Oil flow from the low to high pressure chambers is otherwise permitted. Thus, the lifer is hydraulically rigid against the check valve and competent to open the associated valve and, by accepting oil into the high pressure chamber through the check valve, can extend the operating length of the lifter to remove the accumulated mechanical lash in a valve train while the lifter is on the base circle portion of an associated cam lobe (and thereby eliminating valve clatter and quieting engine operation).
The hydraulic lash compensation mechanism described above must also have the capability to shorten the operating length of the lifter in response to thermally induced dimensional changes in engine components. Thus, in addition to being able to extend the operating length of the lifter as previously described, every lifter must also have the capability of bleeding oil from the high pressure chamber to reduce the operating length of the lifter. As this cannot be done via the check valve, typically a designed-in radial clearance is provided between the wall of the lash compensation mechanism and the surrounding body wall to permit a controlled “leakdown” of oil from the high pressure chamber back to the oil sump of the engine.
The rate of leakdown significantly impacts the amount of valve overlap provided in a combustion cycle. Valve overlap occurs when both the intake and exhaust valve are open within a given cylinder at the same time. Controlling the leakdown and valve overlap is critical because exhaust gas flows from the exhaust system into the intake manifold during the overlap period. The cross-flow of exhaust gas is known in the art as internal exhaust gas recirculation (EGR). Too low of a leakdown rate in an exhaust valve lifter will cause the associated valve to have an increased net lift and a later closing causing greater valve overlap between the opening event of the intake valve and the closing event of the exhaust valve of the same cylinder. The increase in the amount of internal EGR caused by greater valve overlap leads to combustion instability, manifesting as a rough idle. Too high of a leakdown rate in an exhaust valve lifter will cause the valve to have a decreased net lift and an earlier closing causing lesser valve overlap in the associated cylinder and a decreased amount of internal EGR. Leakdown rate variation may occurs both in an intake valve lifter and an exhaust valve lifter. However, since the effect of leakdown in a valve lifter accumulates over the entire lift event, the amount of leakdown when the intake valve is just opening (when valve overlap would occur) and its contribution to varying valve overlap is negligible.
In a DRHVL, the hydraulic lash compensation mechanism resides, slidably, in a bore of the DRHL's locking pin plunger member as shown in FIG. 6 of U.S. Pat. No. 6,814,040. In the case of a DRHVL, the leakdown rate is carefully controlled in manufacture for the purpose of controlling valve overlap variation via control of the clearance between the outer diameter of the mechanism's piston and the inner diameter of the plunger bore. Dimensional variations of these components within an acceptable tolerance range among the manufactured population of pistons and the manufactured population of plungers, result in a random distribution of leakdown rates in the population of DRHVLs and a corresponding distribution of resulting internal EGR.
Another characteristic that affects valve overlap between intake and exhaust valves, but unique to a DRHVL, is the variation in the overall assembled length (working extended length) measured between a point on the DRHVL at which the DRHVL contacts the associated cam lobe and a point on the DRHVL at which it contacts the associated pushrod end or valve stem, when the DRHVL is under valve train load imposed by the associated cam lobe and in valve operating mode (locking pin mechanism engaged). As a starting point, the actual extended length of any DRHVL in a population of DRHVLs must always be shorter than the minimum working distance between a valve stem or pushrod and the base circle portion of an associated cam lobe (minimum available length), to assure that the associated valve can close.
Components within a DRHVL controlling its working extended length and the resulting mechanical lash in the valve train include a radially-extendable locking pin disposed in the plunger member for selectively engaging or disengaging a receiving feature in the lifter body to couple and decouple the plunger to the body and hence to open or, when disengaged, to not open the associated engine valve. A tower coupled at one end to the locking pin plunger member functions at its opposite upper end as a seat for a lost-motion spring. The other end of the lost-motion spring is grounded simultaneously to the lifter body and locking pin plunger member so that, while the DRVHL is in valve-lift mode (plunger coupled to body), and on the base circle of the associated cam lobe, there is a slight clearance between the locking pin and the lower portion of the locking pin receiving feature, referred hereinafter as “G-gap”. As the eccentric portion of the associated cam lobe begins to exert compressive force on the DRVHL to begin opening the associated valve, the first response in the DRVHL is to move against the spring force of the lost motion spring to move the body member upward to take up the G-gap. Thus, depending on the variation in the size of the G-gap, the opening point of the associated valve is delayed as the G-gap is taken up. Further, the total lift of the valve is reduced in proportion to the amount of movement by the body member to take up the G-gap.
The control of G-gap variability from DRVHL to DRVHL is an important characteristic of a DRHVL. Too small of a G-gap (a smaller mechanical lash) in either the intake or exhaust valve DRVHL will result in a greater working extended length and cause the associated valve to open sooner, and to have an increased lift and a later closing causing a longer overlap period and greater internal EGR. Too large of a G-gap (a larger mechanical lash) will shorten the working extended length of the DRVHL and cause the associated valve to have a delayed opening, decreased lift and to close earlier. The result is a shorter period of valve overlap and lesser internal EGR. It is important to note that, since mechanical lash is not accumulative as the lift event progresses, but instead acts over the entire lift event, variations in the G-gap of both the intake valve DRVHL and exhaust valve DRVHL will affect valve overlap and internal EGR within a cylinder and must be carefully controlled.
In order to control the size of the G-gap during assembly of each DRHVL the working extended length of the DRVHL is ascertained, and a shim is inserted from a population of shims of various thicknesses to set the desired G-gap and to bring the assembled length to within tight tolerance limits.
Two problems are presented by this approach in the prior art assembly process of DRVHLs. First, because a finite number of shim thicknesses is available, for practical reasons, the population of lifters has a residual error in actual lengths randomly distributed about a mean. Second, because leakdown rate variation in the hydraulic lash compensation mechanism of a DRVHL is not linked to, and thus occurs independently of variations in the G-gap, the total variation critical to valve lift in a population of DRVHLs is the sum of the independent variations. When DRVHLs having significantly disparate variations in leakdown and mechanical lash, and especially from the extremes of total variation, are employed in different cylinders in the same engine, the differences in valve lift, valve timing, and resulting internal EGR can result in unacceptable cylinder-to-cylinder differences leading to rough idling which is inherent in the engine and cannot be tuned out.
Further, it should be noted that, because leakdown is a time-dependent phenomenon, the tolerance to these variations increases as engine speed increases. Therefore, the troublesome manifestation resulting from a cylinder-to-cylinder variation of internal EGR occurs primarily during engine idle conditions.
What is needed in the art is a means for reducing the variation in the build characteristics of a DRVHL affecting valve lift, in a population of DRVHLs, to reduce the variation of cylinder-to-cylinder internal EGR.
It is a principal object of the present invention to reduce cylinder-to-cylinder variation in internal EGR in an engine and thereby to improve the idling characteristics of an engine.